The present invention relates to the use of naphthalenesulfonic acid-formaldehyde condensation products (condensates) as drying assistants, especially in connection with the spray drying of aqueous polymer dispersions.
The present invention also relates to a process for preparing polymer powders which are redispersible in an aqueous medium, and to the redispersible polymer powders and their use.
Aqueous polymer dispersions are widely employed, for example as binders, especially for synthetic-resin renders or highly pigmented interior paints, adhesives or other coating compositions. In many cases, however, it is desired to use not the aqueous polymer dispersion but the polymer in powder form.
To obtain the polymer in powder form, the dispersion must undergo a drying operation, for example spray drying or else freeze drying. In the case of spray drying, the polymer dispersion is sprayed in a stream of hot air and the water is removed; the air used for drying, and the sprayed dispersion, are preferably passed cocurrently through the dryer.
However, the resulting polymer powder has the disadvantage that its redispersibility in an aqueous medium is generally not entirely satisfactory, since the polymer particle diameter distribution which results from redispersion generally differs from that in the initial aqueous dispersion. The reason for this is that aqueous polymer dispersions, unlike polymer solutions, do not form thermodynamically stable systems. Instead, the system tends to reduce the polymer/dispersion interface by combining small primary particles to form larger secondary particles (gel specks, coagulum). In the state of disperse distribution in the aqueous medium, this can be prevented even for a relatively long time by the addition of dispersants, such as emulsifiers and protective colloids. During the drying of aqueous polymer dispersions, however, the action of the dispersants is in many cases inadequate, and irreversible formation of secondary particles takes place to a certain extent, ie. the secondary particles are retained on redispersion and impair the performance characteristics of the aqueous polymer dispersion that are obtainable on redispersion.
To prevent or at least reduce the formation of secondary particles in the course of drying, it has long been known to employ drying assistants. These substances are widely referred to as spray assistants, since spray drying in particular promotes the formation of irreversibly agglomerated secondary particles. This effect is all the more marked the lower glass transition temperature (and thus the softening point or minimum film-forming temperature) of the polymer particles, especially when it is below the drying temperature. At the same time, drying assistants generally reduce the formation of polymer coating which remains adhering to the dryer wall, and thus result in an increase in powder yield.
The use of drying assistants is known from numerous publications. For instance, DE-A-24 45 813 describes a pulverulent polymer which is redispersible in aqueous systems and whose drying assistant comprises from 1 to 20% by weight of a water-soluble sulfo- or sulfonate-functional condensation product formed from aromatic hydrocarbons and formaldehyde. The condensation products involved are, in particular, condensates of phenolsulfonic or naphthalenesulfonic acid with formaldehyde. There is no information on the molecular weight of the condensates used. It is mentioned that the drying of the polymer powders should be undertaken at below the softening point.
EP-A-78 449 describes a process for preparing water-redispersible, blocking-resistant polymer powders by spray drying aqueous dispersions of polymers having glass transition temperatures below 50xc2x0 C. As spray assistant the dispersions include a water-soluble copolymer of vinylpyrrolidone and vinyl acetate and/or a water-soluble alkali metal salt and/or alkaline earth metal salt of a naphthalenesulfonic acid-formaldehyde condensate. Here again, there is no information on the molecular weight of the naphthalenesulfonic acid-formaldehyde condensates employed. What is evident is the comparatively large amount of spray assistant in the case where the naphthalenesulfonic acid-formaldehyde condensates are used alone (30% by weight in Ex. 4, 50% by weight in Ex. 5, 30% by weight in Ex. 6, based in each case on the polymers). This adversely affects the binder properties of the polymer powders for example by increasing by an undesirable extent the flowability of compositions in which they are used as binder (cf. EP 407 889), or by retarding the setting of cementitious compositions.
Similarly, EP-A-407 889 describes the use of a water-soluble alkali metal salt or alkaline earth metal salt of a phenolsulfonic acid-formaldehyde condensate as a spraying assistant for preparing water-redispersible polymer powders from aqueous polymer dispersions. Yet again, there is no information on the molecular weight of the condensates used.
It is an object of the present invention to provide drying assistants which permit the preparation, from polymer dispersions, of polymer powders which are readily redispersible in water and which do not have the disadvantages of the prior art.
We have found that this object is achieved, surprisingly, by using naphthalenesulfonic acid-formaldehyde condensates having a number-average molecular weight Mn less than 1500 as drying assistants.
The present invention therefore relates to the use of naphthalenesulfonic acid-formaldehyde condensates having a number-average molecular weight Mn less than 1500 daltons or salts thereof as drying assistants in the drying of aqueous polymer dispersions.
The condensates preferably have mean molecular weights Mn in the range from 500 to 1500, preferably from 700 to 1250 daltons, determined by means of gel permeation chromatography, as described in the Examples for the preparation of the spraying assistants. The molecular weight distribution or polydispersity (defined as Mw/Mn) is in the range from 5 to 15, preferably from 5 to 10. The proportion of condensates with molar masses above 10,000 daltons is preferably less than 25% by weight, in particular less than 20% by weight, of the overall condensate.
The condensates used should generally comprise no more than 25% by weight, preferably no more than 15% by weight, of unreacted naphthalenesulfonic acid (xcex1 and/or xcex2 product) and not less than 75% by weight of condensates. Where the condensate is employed in the form of its salts, the salts normally used are alkali metal salts or alkaline earth metal salts or ammonium salts, ie. salts with ammonia or with organic amines such as triethanolamine, diethanolamine and triethylamine. Preference is given to the alkaline earth metal salts, and especially to the calcium salts.
The drying assistants employed in accordance with the invention are generally prepared by condensation of naphthalenesulfonic acid with formaldehyde under acidicxe2x80x94especially sulfuricxe2x80x94reaction conditions. In this reaction the naphthalenesulfonic acid can be introduced as initial charge or can be prepared in situ by sulfonation in accordance with known methods (cf. J. March, Advanced Organic Chemistry, 3rd ed., John Wiley, New York 1985, p 473 ff. and literature cited therein). Naphthalenesulfonic acid is preferably prepared in situ by sulfonation with sulfuric acid, preferably concentrated sulfuric acid. Condensation takes place by reacting naphthalenesulfonic acid with formaldehyde under acidic reaction conditions, preferably under sulfuric reaction conditions, and especially in concentrated sulfuric acid. Where the naphthalene acid is prepared in situ, the condensation reaction is initiated by adding formaldehyde to the sulfuric reaction mixture. The molar ratio of formaldehyde to naphthalenesulfonic acid is in the range from 1:1 to 1:2, preferably 1:1.3 to 1:1.7. Formaldehyde is preferably added in the form of an aqueous solution. In order to establish the desired molecular weight, the condensation reaction is generally conducted at from 90 to 110xc2x0 C., preferably at about 100xc2x0 C. The reaction time is generally from 3 to 8 h, preferably from 4 to 6 h. Where it is desired to use the salts as drying assistants, condensation is followed by neutralization using an appropriate basic metal salt or an amine, in which case either metal salt or amine are used preferably in the form of an aqueous solution or dispersion.
The present invention also provides a process for preparing a polymer powder by drying an aqueous polymer dispersion, which involves using at least one naphthalenesulfonic acid-formaldehyde condensate of the type described above, or a salt thereof, as drying assistant. In the case of alkaline polymer dispersions, the condensates are employed in salt form, but are employed in acid form in the case of acidic polymers.
The amount of drying assistant used is preferably from 1 to 30% by weight, based on the weight of the polymer in the dispersion, preferably from 3 to 20% by weight and, with particular preference, from 5 to 15% by weight.
With particular advantage the novel compounds are used to dry dispersions of polymers whose glass transition temperature (DSC, midpoint temperature, ASTM D 3418-82) is xe2x89xa665xc2x0 C., preferably xe2x89xa650xc2x0 C., particularly preferably xe2x89xa625xc2x0 C. and, with very particular preference, xe2x89xa60xc2x0 C. The glass transition temperature of the polymers is generally xe2x89xa7xe2x88x9260xc2x0 C., preferably 24 xe2x88x9240xc2x0 C. and, in particular, 24 xe2x88x9220xc2x0 C.
Against this background it is often useful to estimate the glass transition temperature Tg of the dispersed polymer. According to Fox (T.G. Fox, Bull. Am. Phys. Soc. (Ser. II) 1, 123 [1956] and Ullmanns Encyklopadie der technischen Chemie, Weinheim (1980), p. 17, 18), for the glass transition temperature of copolymers of high molar mass, it holds in good approximation that       1          T      g        =                    X        1                    T        g        1              +                  X        2                    T        g        2              +          ⋯      ⁢                        X          n                          T          g          n                    
where X1, X2, . . . , Xn are the mass fractions 1, 2, . . . , n and Tg1, Tg2, . . . , Tgn are the glass transition temperatures, in kelvins, of homopolymers of each of the monomers 1, 2, . . . , n. The individual Tg5 are known, for example, from Ullmann""s Encyclopedia of Industrial Chemistry, VCH, Weinheim, Vol. A 21 (1992) p. 169 and from J. Brandrup, E. H. Immergut, Polymer Handbook 3rd ed., J. Wiley, New York 1989.
Preferred polymers are those composed of:
(a) from 80 to 100% by weight of at least one monomer selected from vinyl-aromatic compounds, esters of xcex1,xcex2-monoethylenically unsaturated C3-C6-carboxylic acids and C1-C12-alkanols, preferably C1-C8-alkanols, vinyl esters and allyl esters of C1-C12-carboxylic acids, and butadiene, and
(b) from 0 to 20% by weight of at least one other monomer which has at least one ethylenically unsaturated group.
Here, the expressions Cn-Cm refer to the possiblexe2x80x94for the purposes of the inventionxe2x80x94number of carbons in a particular class of compound. Alkyls may be linear or branched. Cn-Cm-Alkylaryl are aryls which carry a Cn-Cm-alkyl.
Examples of vinyl-aromatic compounds are styrene, xcex1-methylstyrene and vinyltoluenes, such as o-vinyltoluene.
The esters of xcex1,xcex2-monoethylenically unsaturated carboxylic acids are, in particular, esters of acrylic, methacrylic, maleic, fumaric and itaconic acid. Examples of such esters are methyl, ethyl, n-butyl, isobutyl, t-butyl, ethylhexyl, decyl and dodecyl (meth)acrylate, dimethyl maleinate, di-n-butylmaleinate and di-n-butylfumarate.
Vinyl and alkyl esters which can be used are vinyl acetate, propionate, n-butyrate, laurate and stearate and the corresponding allyl esters.
Particularly preferred monomers (a) are n-butyl acrylate, tert.-butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate tert.-butyl methacrylate, vinyl acetate, vinyl propionate, butadiene and styrene.
The monomers (b) preferably comprise monomers (bxe2x80x2) having an increased water solubility. They are usually selected from the abovementioned xcex1,xcex2-monoethylenically unsaturated C3-C6-carboxylic acids and the nitriles, amides, monoalkylamides, dialkylamides, N-alkylolamides and hydroxyalkyl esters thereof. It is also possible to use the N-vinyl derivatives of cyclic lactams and the mono- or dialkylaminoalkylamides of the abovementioned C3-C6-carboxylic acids and quaternization products thereof.
Particularly preferred monomers (b) are acrylamide, methacrylamide, acrylic acid, methacrylic acid, acrylonitrile, methacrylonitrile, 2-acrylamido-2-methylpropanesulfonic acid, vinylpyrrolidone, N-methylolacrylamide, N-methylolmethacrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, quaternized vinyl-imidazole, N,N-dialkylaminoalkyl (methy)acrylates, N,N-dialkyl-aminoalkyl(meth)acrylamides, trialkylammoniumalkyl (meth)acryl-ates and trialkylammoniumalkyl(meth)acrylamides.
The polymers may also include further monomers (bxe2x80x3) in order to give the respective polymer films greater strength. Monomers (bxe2x80x3) comprise compounds which have at least two non-conjugated double bonds. Examples of such compounds are the diesters of dihydric alcohols with xcex1,xcex2-monoethylenically unsaturated acids such as ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,3-butylene and 1,4-butylene glycol di(meth)acrylate and 2,2-dimethyl propylene glycol di(meth)acrylate, the vinyl and allyl esters of dicarboxylic acids, such as divinyl and diallyl maleate, divinyl and diallyl fumarate, divinyl and diallyl phthalate, divinyl benzene, cyclopentadienylacrylate and -methacrylate, tricyclodecenyl(meth)acrylate, N,Nxe2x80x2-divinyl imidazolin-2-one and triallyl cyanurate. Such compounds are generally used in amounts up to 5% by weight of the total monomers.
Further examples of these are siloxane-functional monomers, such as the vinyltrialkoxysilanes, for example vinyltrimethoxysilane, alkylvinyldialkoxysilanes or (meth)acryloxyalkyltrialkoxysilanes, for example (meth)acryloxyethyltrimethoxysilane and (meth)acryloxypropyltrimethoxysilane. These siloxane monomers can be used in amounts up to 2 parts by weight, preferably from 0.05 to 1 part by weight, based on 100 parts by total weight of monomers.
Preferred polymer dispersions, moreover, are those in which the weight-average diameter dw of the dispersed polymer particles is xe2x89xa7100 nm, particularly preferably xe2x89xa7300 nm. Normally, dw is xe2x89xa72000 nm. It is also favorable if the diameters of the dispersed polymer particles are spread over a broad diameter range.
The dw of the particle size is, conventionally, defined as the weight average of the particle size, as determined using an analytical ultracentrifuge in accordance with the method of W. Scholtan and H. Lange, Kolloid-Z. und Z.-Polymere 250 (1972) pp. 782-796. The ultracentrifuge measurement provides the integral mass distribution of the particle diameter of a sample. From this it is possible to derive what percentage by weight of the particles have a diameter equal to or less than a particular size.
An appropriate measure of the width of the diameter distribution is the quotient Q=(d90-d10)/d50, where dm is the diameter not exceeded by m% by weight of the dispersed polymer particles. Q is preferably from 0.5 to 1.5. The preparation of polymer dispersions having such a breadth of particle distribution is familiar to the skilled worker, for example from DE-A-43 07 683.
The ratio of weight-average molecular weight MW to number-average molecular weight Mn of the polymers can be from 1 to 30 or from 1 to 20 or from 1 to 8. Accordingly, the molecular weight can be essentially uniform or can be distributed over a certain breadth.
The preparation of the polymer dispersions that are to be dried is known. They are generally prepared by free-radical polymerization, preferably in polar solvents and especially in water. To establish the desired molecular weight it is possible to add substances which regulate the molecular weight. Examples of suitable molecular weight regulators are compounds containing a thiol group and/or a silane group (examples being t-dodecyl mercaptan, n-dodecyl mercaptan and mercaptopropyltrimethoxysilane), allyl alcohols and aldehydes, such as formaldehyde, acetaldehyde, etc.
Examples of suitable initiators are inorganic peroxides, such as sodium peroxodisulfate, or azo compounds. Solution or emulsion polymerization can be carried out depending on the monomer composition.
If the polymer dispersion is prepared by emulsion polymerization, it is carried out conventionally. A protective colloid is generally used, such as polyvinyl alcohol, polyvinyl pyrrolidone or cellulose derivatives, or anionic and/or nonionic emulsifiers, such as ethoxylated mono-, di- or trialkylphenols, ethoxylated fatty alcohols, and alkali metal or ammonium salts of C8-C12-alkyl sulfates, sulfuric monoesters of ethoxylated C12-C18-alkanols, C12-C18-alkylsulfonic acids, C9-C18-alkylarylsulfonic acids and sulfonated alkyldiphenyl ethers. Polymerization is generally carried out at from 50 to 120xc2x0 C., in particular from 70 to 100xc2x0 C.
The emulsion polymerization might be carried out by the seed latex technique in order to adjust particle size and particle size distribution. The seed latex might be preformed or be prepared in situ. Techniques for doing this are known and can be found in prior art (see EP-B 40 419, EP 129 699, EP-A 567 811, EP-A 567 812, EP-A 567 819, DE-A 31 47 008, DE-A 42 13 967 and DE-A 42 13 968 which are incorporated herein by reference). In another embodiment of the present invention the polymer dispersion is carried out in the absence of a seed latex. In this case, particle size can be adjusted by surface active substances such as protective colloids or emulsifiers.
The dispersion itself may be a primary dispersion, ie. a polymer dispersion obtained directly by the method of free-radical aqueous emulsion polymerization. Alternatively, it may be a secondary dispersion; in other words, a polymer obtained by solution polymerization is subsequently converted into an aqueous polymer dispersion.
The polymer dispersion can be dried in a customary manner, for example by freeze drying or, preferably, by spray drying. In the case of spray drying, a procedure is used in which the stream of hot air enters at from 100 to 200xc2x0 C., preferably from 120 to 160xc2x0 C., and exits at from 30 to 90xc2x0 C., preferably from 60 to 80xc2x0 C. Spraying the aqueous polymer dispersion in the stream of hot air can be effected, for example, by means of single-substance or multi-substance nozzles or using a rotating disk. The polymer powders are normally separated off using cyclones or filter separators. The sprayed aqueous polymer dispersion and the stream of hot air are preferably conducted in parallel.
The naphthalenesulfonic acid-formaldehyde condensates used in accordance with the invention can be added in the form of an aqueous solution or as a solid, prior to drying, to the dispersion that is to be dried. Where the latter is a primary dispersion, the drying assistant can be added before, during and/or after emulsion polymerization.
In addition to the novel drying assistants it is also possible, as well, to use known drying assistants, such as polyvinyl alcohol, polyvinyl pyrrolidone, phenolsulfonic acid-formaldehyde condensates, homopolymers of 2-acrvlamido-2-methylpropanesulfonic acid, etc. Anticaking agents, such as highly disperse silica, which is normally used for the drying of aqueous polymer dispersions, may be employed in order to prevent the polymer powder caking together in the course of storage. In the case of spray drying, the anticaking agents are generally fed in through a separate nozzle.
The present invention also provides polymer powders obtainable in accordance with the invention. These are suitable as binders in hydraulically setting compositions, paints, lacquers, adhesives, coating compositions (especially for paper) and synthetic-resin renders, as are described in EP-A-629 650.
The polymer powders obtainable in accordance with the invention are particularly suitable for the modification of binding mineral building materials (mortar-like formulations) which comprise a mineral binder consisting of 70-100% by weight cement and 0-30% by weight gypsum. In this context, the novel effect is essentially independent of the type of cement. Depending on the project at hand, therefore, it is possible to use blast furnace slag cement, oil shale cement, Portland cement, hydrophobicized Portland cement, quick-setting cement, high-expansion cement or high-alumina cement, the use of Portland cement proving particularly advantageous. See DE-A 19623413.3 for further details.
The dry compositions of binding mineral building materials typically include, based on the amount of mineral binder, from 0.1 to 20% by weight of modifying polymer powder.
Cellulose derivatives and microsilica are frequently added to the binding mineral building materials in order to improve their processing properties. The former additives usually have a thickening action, while the latter normally constitute thixotropic agents which bring about a further reduction in the flowability of the aqueous mortar prior to its solidification in the applied, at-rest state. Calcium carbonate and quartz sand generally make up the remaining aggregates. By adding defoamers (preferably in powder form in the context of dry mortars) it is possible to obtain, in the solidified state, a solidified cementitious mortar with an air pore content appropriate to the field of use (from 5 to 20% by volume).
The polymer powders obtainable in accordance with the invention are suitable, for example, for modifying cementitious repair mortars or reinforcing mortars. In this context, customary reinforcing mortars additionally include natural or synthetic fibers of materials such as, for example, Dralon (length for example from 1 to 10 mm, linear density for example from 3 to 10 dtex) in order to increase their crack-bridging capability.
The amount of modifying polymer powder added to the cementitious reinforcing mortar will be, based on cement content, from 9 to 20% by weight for extremely high crack-bridging requirements and from 4 to 9% by weight for lesser crack-bridging requirements. Only in the case of particularly low crack-bridging requirements will the amount of modifying polymer powder added be limited, again based on cement content, to the range from 0.1 to 4% by weight.
Typical reinforcing mortars consist, as a dry preparation of binding mineral building material, of from 20 to 60, preferably from 20 to 50, % by weight of mineral binder (preferably exclusively cement), from 0.1 to 20, frequently from 0.1 to 10, % by weight of modifying polymer powder obtainable in accordance with the invention, up to 25% by weight of customary auxiliaries (such as defoamers or thickeners) and, as the remainder, aggregates such as, for example, sand, fillers (eg. CaCo3), pigments (eg. TiO2) and natural and/or synthetic fibers.
The Examples which follow illustrate the invention without restricting it.